The availability of frequency resources has become tight recently, and therefore digital-based high-performance transmittance are being used more and more in the field of wireless communications. A typical high-performance transmission is made using a multilevel QAM (quadrature amplitude modulation) method and an amplifier having high linearity is requested. To reduce the size and power consumption of a wireless transmitting device, an amplifier having high performance operation is requested. As a means of realizing a high-performance linear amplifier, a radio frequency amplifier circuit is proposed that uses a saturated amplifier incorporating LINC (Linear Amplification with Nonlinear Components).
The operation of a conventional radio frequency amplifier circuit using LINC (hereinafter, “LINC-based amplifier circuit”) will be explained below. Upon receiving an input signal Sin(t) having envelope variation, the LINC-based amplifier circuit separates the input signal Sin(t) into two phase modulation signals Sc1(t) and Sc2(t) in such a manner that their phase difference is set in accordance with the amplitude and then outputs Sc1(t) and Sc2(t). The signals that are generated by separating the input signal are sometimes called “branch signals”. Then, Sc1(t) is converted to an analog signal and the analog signal passes through a filter. After passing through the filter, unnecessary frequency components are suppressed and the component that corresponds to Sc1(t) is extracted. The LINC-based amplifier circuit orthogonally modulates the extracted component that corresponds to Sc1(t), thereby generating a radio frequency signal S1(t) that is an RF signal. The LINC-based amplifier circuit processes Sc2(t) in the same manner, thereby generating S2(t), which is also an RF signal. The LINC-based amplifier circuit includes parallel amplifiers that correspond to S1(t) and S2(t), respectively. S1(t) and S2(t) are amplified by the respective amplifiers, each having the gain G. In this circuit, the amplifiers are used as saturated amplifiers. With this configuration, the respective amplifiers generate amplified radio frequency signals GS1(t) and GS2(t), respectively. After that, the LINC-based amplifier circuit combines the radio frequency signals GS1(t) and GS2(t) together, thereby generating an output radio frequency signal Sout(t). The LINC-based amplifier circuit then outputs the generated Sout(t). As described above, the LINC amplifier circuit generates the output radio frequency signal Sout(t) by amplifying the input signal Sin(t) or performs efficient signal amplification.
Examples of a signal separating method used in a conventional LINC-based amplifier circuit include a method of separating a signal using the following equation 1. In Equation 1, the input signal is expressed in polar coordinates, where x corresponds to Sin(t) in the above description, xa is Sc1 in the above description, and xb is Sc2 in the above description:x=r·ejθ(r≦1)xa=ej(θ+cos−1(r)) xb=ej(θ−cos−1(r))  (1)
The symbol r is the amplitude of the input signal. The symbol θ represents the modulated phase of the input signal. The phase angle of the branch signal xa and the phase angle of the branch signal xb are set in such a manner that the difference between the phase of the branch signal xa and the phase of the branch signal xb becomes 2×cos−1(r).
FIG. 10 is a constellation diagram of conventional branch signals when the input signal is a two-tone signal. The constellation is generated by plotting a signal in two-dimensional coordinates. The vertical axis of FIG. 10 is a Q component, and the horizontal axis is an I component. The branch signal xa is a signal 901 expressed by a solid line, and the branch signal xb is a signal 902 that is expressed by a dotted line.
A proposed conventional technology relates to a LINC-based amplifier circuit that can amplify a signal with a high efficiency even when the dynamic range is wide and the PAR is high. The conventional LINC-based amplifier circuit calculates the amplitude of a branch signal within a certain interval of an input signal by using the average power within the interval and the maximum power within the interval.
Patent Document 1: Japanese Laid-open Patent Publication No. 2008-28509
If a phase reversal signal, such as a PSK signal, is input to a LINC-based amplifier circuit as an input modulation signal x, branch signals xa and xb that are generated by separating the signal have 180-degrees reversed parts. In other words, the generated branch signals xa and xb have discontinuous parts, which largely increase the bandwidths of the branch signals xa and xb. For example, in FIG. 10, parts close to a point 903 are the 180-degrees reversed parts.
However, because of the Nyquist theorem, it's difficult for the digital branch signals xa and xb to be expressed by using a frequency that is more than half of the sampling frequency. Therefore, when the branch signals illustrated in FIG. 10 are converted by D/A (Digital/Analog) converters into analog signals and then folding components are removed therefrom by using smoothing filters, a signal 904, which is expressed by a solid line in FIG. 11, and a signal 905, which is expressed by a dotted line in FIG. 11, are generated. The signals 904 and 905 are input to respective amplifiers. FIG. 11 is a constellation diagram of conventional signals that are input to amplifiers. The vertical axis of FIG. 11 is the Q component, and the horizontal axis is the I component. As illustrated in FIG. 11, it is found that both the signals 904 and 905 are wavy and large ringing occurs in each of the signals that are input to the amplifiers. In other words, the signals that are input to the amplifiers are different from constant envelope signals. The ringing occurs due to the effect of the 180-degrees reversed parts, such as the point 903. In this example, the signals that are input to the amplifiers include amplitude components due to the effect of the ringing. When a signal having an amplitude component is amplified by an amplifier, due to effects of AM/AM distortion and AM/PM distortion of the amplifier, the combined output radio frequency signal Xout is deteriorated and, therefore, distortion occurs. FIG. 12 is a constellation diagram of conventional signals that are output from the amplifiers and a conventional combined signal. The vertical axis of FIG. 12 is the Q component and the horizontal axis is the I component. The signals that are output from the amplifiers are a signal 906, which is expressed by a solid line, and a signal 907, which is expressed by a dotted line. The combined signal of the signals 906 and 907 is a signal 908, which is expressed by a dashed-dotted line. As illustrated in FIG. 12, distortion occurs in each of the signals 906, 907, and 908.
Because the distortion components depend on the frequencies of the signals that are input to the amplifiers, it is difficult to compensate for them by using a conventional nonlinear compensation circuit (linearizer). Moreover, because the signals that are input to the amplifiers have wide frequency bands, it is difficult to add compensation circuits to the respective amplifiers to compensate for the nonlinear characteristics. FIG. 13 is a constellation diagram of signals that are output from the amplifiers and a conventional combined signal when the conventional LINC-based amplifier circuit additionally has nonlinear compensation circuits. The vertical axis of FIG. 13 is the Q component and the horizontal axis is the I component. The signals that are output from the amplifiers are a signal 909, which is expressed by a solid line, and a signal 910, which is expressed by a dotted line. The combined signal of the signals 909 and 910 is a signal 911, which is expressed by a dashed-dotted line. As illustrated in FIG. 13, distortion occurs in each of the signals 909, 910, and 911.
As described above, because distortion occurs when the conventional signal separating method is used, it is difficult to suppress deterioration of a combined output or an output radio frequency signal.
Moreover, even if the conventional technology is used and the amplitude of a branch signal within a certain interval of an input signal is calculated by using the average power within the interval and the maximum power within the interval, it is still difficult to avoid the occurrence of a discontinuous branch signal and to suppress the occurrence of distortion.